PRDM9 and atypical Rett syndrome: A previous report demonstrated that G9a histone methyltransferase elevates the density of dendritic spines in nucleus accumbens neurons by reducing H3K9me2 levels on exposure to cocaine.45 In addition, mutation of MeCP2, the causative gene of Rett syndrome and an epigenetic regulator that acts by binding to methylated DNA, also causes a decrease in the number of dendritic spines.46 Thus, there is increasing evidence that epigenetic mechanisms affect the development and plasticity of dendritic spines.